Do I have to use CoffeeScript?
While composing in CoffeeScript is obviously the point, you could store the compiled javascript version of the
template, and after that no coffee-script dependency is required. There is a pure javascript version of
the engine provided, as well.

Does it only work in Node.js or client-side/in-browser as well?
It is designed to be used on either side. This is why it is lightweight, with no dependencies.